The electrophoresis of electrostatically-charged species toward an interface between two materials, in order to modify the effective refractive index value in the region near the interface is known. Such electrophoresis may cause a change in the behavior of incident light rays. This change in the behavior of incident light rays has been investigated with regard to creating high reflectance image displays based on total internal reflection of the type described in U.S. Pat. Nos. 6,885,496 and 6,891,658. Generally, the charged electrophoretic materials are pigments.
One problem when the electrophoretic species are pigment particles is that the electrophoretic particles exhibit undesirable clustering over time. More particularly, the particles tend to form loose agglomerates within the electrophoretic medium, while surrounding regions of the electrophoretic medium contain relatively few suspended particles. Such clustering of absorptive particles can cause long-term deterioration of the performance of the device as described, for example, in “The understanding and elimination of some instabilities in an electrophoretic display”, P. Murau and B. Singer, Journal of Applied Physics, Vol. 49(9), 1978. For this reason, although considered, modulation of diffraction gratings using electrophoresis of pigment particles has not been pursued. (See “Electrical modulation of diffractive structures”, V. Kwong, M. Mossman and L. Whitehead, Applied Optics, Vol 41(16), 3343-3347, 2002.)